The invention relates to compound lenses and more particularly to compound lenses incorporating ordinary crown glass and calcium fluoride elements providing improved optical characteristics.
For many years an urgent problem in lens design has been needed correction of secondary spectrum in large optical systems of moderate aperture and field in which all other aberrations can be virtually eliminated, as noted by J. G. Baker, Appl. Opt. 2, 111-129 (1963). Many patents show the use of calcium fluoride (CaF.sub.2) crystal in conjunction with various glasses to solve the problem. For example, U.S. Pat. No. 2,455,808 to Reiss shows a well known lens type (the Double Gauss) in combination with CaF.sub.2 in two elements and glass of refractive index less than 1.57 for the remaining elements. The extent to which this solution satisfies the goal is not specified. The glasses used are all outside the ranges of those of the instant invention.
U.S. Pat. No. 2,487,873 to Hertzberger et al. discloses a triplet design with some performance results given in the table in Col. 4, line 50. The paraxial secondary spectrum for the C-F range calculates to be 1 part in 5300 and spherochromatism at f/7.1 calculates as 1 part in 5500. The lens of this patent would not be good for more than a 2.degree. field because it does not produce good correction of other aberrations. The allegation of "almost perfect correction of secondary spectrum" (Col. 4, line 34) does not seem to be borne out by the numbers in the table. In any case, the glass used, 620604, is far outside the glass index range of the instant invention. Example (3) uses glass 541599, which is at the edge of the range of the instant invention but not in the best region. (Col. 5, line 73).
The evaluation of secondary spectrum by paraxial ray tracing is the classical procedure because the calculations are well suited for the pencil and paper methods used a century ago. When mechanical calculators became commercially available (1930), the more tedious spherochromatism calculations for the whole lens aperture came into use because it gave a better evaluation of lens performance. When electronic computers became commercially available (1950), the positions of the best image spots throughout the field gave an even better evaluation of secondary chromatic errors, along with a better evaluation of all aberrations. The LASL lens design code disclosed in LA-UR-73-286 uses the image-spot size and position data to evaluate lens performance.